1. Field of the Invention
This invention relates to thermoplastic compositions, and more particularly to polyester including polycarbonate compositions having improved ductility or toughness, and to the processes for preparing such compositions.
2. Description of the Prior Art
Unmodified thermoplastic polyesters are frequently regarded as "tough". For example, some polyesters have good elongation; high energy to break, as demonstrated in tensile tests; high tensile impact strength and high energy absorption as demonstrated in falling dart tests, e.g., the Gardner impact test. In one aspect of toughness the polyester materials are quite erratic; namely, resistance to crack propagation. This deficiency is reflected in notch sensitivity, brittle breaks and occasional catastrophic failure of molded or extruded parts. The tendency of some polyesters to break in a brittle rather than ductile fashion is a significant limitation of utility. A resin may be characterized in its tendency toward ductility by the notched Izod test ASTM D-256-56. With the normal notch radius of 10 mils, polyethylene terephthalate and polybutylene terephthalate will have notched Izod values of about 0.3 to 1 ft. lb./inch of notch. Polyesters known as polycarbonates may have values in the range of 10 to 15 ft. lbs./inch of notch in 1/8 inch sections, but the value falls off rapidly when the notch radius is decreased or thicker samples are used.
There is much prior art concerned with improving the impact strength of polyesters. A variety of additives have been added to polyesters with some improvement in toughness being obtained. British Pat. No. 1,208,585 discloses that a notched Izod of 4.3 ft. lbs./inch can be achieved with the addition of 25 percent by weight terpolymer to the polyester. British Pat. No. 1,208,585 fails to recognize that combination of properties necessary to achieve high toughess. For example, particle size ranges from 0.5 to 10 microns; if the size is too small the improvement in impact strength may be reduced. In addition, the material which causes the adherence is not present in the phase with the at least one polymer.
Cope U.S. Pat. No. 3,435,093 discloses blends of polyethylene terephthalate and an ionic hydrocarbon copolymer of .alpha.-olefins of the formula R--CH.dbd.CH.sub.2 where R is hydrogen or an alkyl radical of 1 to 3 carbon atoms and .alpha.,.beta.-ethylenically unsaturated carboxylic acids containing 3 to 5 carbon atoms, the carboxylic acid groups of the ionic copolymer being 0 to 100 percent neutralized by metal cations. Cope states that improvement in toughness is achieved by the blends. Cope, however, did not recognize that improved toughness of polyester compositions can be achieved provided that at least one polymer present has a small particle size, a tensile modulus of 20,000 or less and the ratio of the tensile modulus of the polyester to the tensile modulus of said polymer is greater than 10 to 1.
Gander et al. U.S. application Ser. No. 488,826, filed July 15, 1974, discloses a polyethylene terephthalate/terpolymer blend, the terpolymer being present in 1 to 5 percent by weight and comprising at least 65 percent by weight ethylene with (a) at least 5 percent by weight of vinyl ester of a 1 to 6 carbon atom saturated monobasic aliphatic carboxylic acid and (b) 0.5 to 10 percent by weight of acrylic or methacrylic acid. It is disclosed that polyethylene terephthalate blended with the terpolymer is tough. It is not taught that improved toughness can be achieved in polyesters if the particle size, tensile modulus of the at least one polymer and the ratio of tensile modulus of the at least one polymer and the polyester matrix resin are maintained within certain ranges.
French Pat. No. 2,194,741 relates to blends of polyesters melting above 100.degree. C. with 1.5 to 90 percent by weight of a copolyether ester. Impact strength is said to be improved. The copolyether esters are block copolymers rather than random copolymers. There is no teaching that tensile modulus, ratios of tensile modulus and particle size of the polymer additive are necessary to provide improved toughness.
Polycarbonates are known to be among the tougher materials used for molded and extruded parts. Kim U.S. Pat. No. 3,334,154 is an example. It has been found, however, that when polycarbonates have been tested for toughness using the notched Izod test as indicated above the toughness is a function of the notch radius. This disadvantage is substantially eliminated when at least one polymer according to this invention is present with the polycarbonate.